1. Field of the Invention
This invention relates generally to a method and apparatus for removing tack welds from reactor vessel components via electro-discharge machining (EDM).
2. Description of Related Art
Electrical Discharge Machining (EDM) is a common technique used for machining hard metals or those that would be impossible to machine with traditional techniques. EDM may be well-suited for cutting intricate contours or delicate cavities that would be difficult to produce with a grinder, an end mill or other cutting tools. EDM removes metal by producing a rapid series of repetitive electrical discharges. These electrical discharges are passes between an electrode and the piece of metal being machined. The repetitive discharges may remove the unwanted metal or create a set of successively deeper craters in the metal until a final shape is produced. Because EDM works with materials that are electrically conductive, metals that can be machined may include, for example, but not limited to, hastalloy, hardened tool-steel, titanium, carbide, inconel and kovar.
EDM is used to machine metals in a reactor pressure vessel (RPV) of a boiling water reactor (BWR). A typical reactor pressure vessel at a BWR has a generally cylindrical shape and is closed at both ends, e.g., by a bottom head and a removable top head. A top guide typically is spaced above a core plate within the RPV. A core shroud, or shroud, typically surrounds the core and is supported by a shroud support structure. Particularly, the shroud has a generally cylindrical shape and surrounds both the core plate and the top guide. There is a space or annulus located between the cylindrical reactor pressure vessel and the cylindrically-shaped shroud.
FIG. 1 is a schematic, partial cross sectional view, with parts cut away, of a reactor pressure vessel (RPV) 20 for a boiling water reactor. The RPV 20 has a generally cylindrical-shape and is closed at one end by a bottom head and at its other end by removable top head (not shown). A top guide (not shown) is situated above a core plate 22 within RPV 20. A shroud 24 surrounds core plate 22 and is supported by a shroud support structure 26. A downcomer annulus 28 is formed between the shroud 24 and a sidewall 30 of RPV 20.
An annulet nozzle 32 extends through sidewall 30 of the RPV 20 and is coupled to a jet pump assembly 34. The jet pump assembly 34 includes a thermal sleeve 36 which extends through nozzle 32, a lower elbow (only partially visible in FIG. 1), an inlet riser pipe 38 coupled to a pair of jet pumps 35, and a jet pump riser brace assembly 40. Thermal sleeve 36 is secured at a first end (not shown) to a second end of the lower elbow. The first end of thermal sleeve 36 is welded to the second end of the lower elbow. A first end of the lower elbow is similarly secured or welded to one end of the riser pipe 38. The riser pipe 38 extends between and substantially parallel to the shroud 24 and sidewall 30. The jet pumps 35 are circumferentially distributed around the core shroud 24. The riser pipe 38 is coupled to the two jet pumps 35 by a transition assembly 39.
FIG. 2 is a perspective view of a jet pump assembly 34. Each jet pump 35 includes a jet pump nozzle 64, a suction inlet 66, an inlet mixer 41, and a diffuser 42. The jet pump nozzle 64 may be positioned in the suction inlet 66 which may be located at a first end (not shown) of inlet mixer 41. The transition assembly 39 may include a base piece 70 and two elbows. Each elbow is coupled to a jet pump nozzle 64. Locking arms 72, 74, 76, and 78 extend from the transition assembly base piece 70. Jet pump beams 86 are connected between the locking arms 72, 74, 76 and 78. One jet pump beam 86 is engaged to a first pair of locking arms 72 and 76, and another jet pump beam 86 is engaged to a second pair of locking arms 74 and 78, as shown in FIG. 2. Each jet pump beam 86 includes a tongue member 81 at an end thereof which engages notches 82 in the locking arms 72, 74, 76 and 78 for preventing and/or reducing movement (e.g., rotational) of a corresponding beam bolt 94. The jet pump beams 86 engage locking arms 72, 74, 76 and 78 by sliding the tongue member 81 into the notches 82.
The jet pump beams 86 are generally attached to the inlet mixer 41 by a retainer plate (not shown) which is locked to the inlet mixer 41 with a retainer bolt 90. To lock the retainer bolt 90, tack welds are applied to prevent the retainer bolt 90 from vibrating loose. However, jet pump beams 86 occasionally need to be repaired or replaced. Thus, in order to repair/replace the jet pump beams 86, the entire inlet mixer 41 is removed and moved into an open area to allow better access to separate the jet pump beams 86 from the inlet mixer 41 (e.g., by breaking the tack welds 91, removing the retainer bolt 90 and removing the retainer plate). However, this procedure is time consuming, labor intensive, and requires additional equipment on-site.
Other techniques of breaking the tack welds 91 and removing the retainer bolt 90 use a torque multiplier to remove the tack welds 91. However, torque multipliers are bulky and large, and thus cannot fit in the area of the jet pump beams 86.